A DPF system that reduces an amount of PM discharged to the outside by collecting PM contained in exhaust gas from a diesel engine in a filter known as a DPF, for example a DPD (Diesel Particulate Defuser) serving as a type of DPF, has been developed.
Similarly to other exhaust gas purification systems (an SCR (Selective Catalytic Reduction) system, for example), a DPF system employing a DPF is provided midway in an exhaust pipe in order to purify exhaust gas and discharge the purified exhaust gas into the atmosphere (see Patent Document 1, for example).
A continuous regeneration type DPF system in which a DOC (Diesel Oxidation catalyst) is provided on an upstream side of the DPF is available as a DPF system. This DPF system makes use of the fact that PM oxidation using NO2 is performed at a low temperature by oxidizing the PM using O2 in the exhaust gas, and performs DPF regeneration by oxidizing NO in the exhaust gas into NO2 using the upstream side DOC and oxidizing PM collected in the downstream side DPF into CO2 using the NO2, whereby the PM is removed.
In this type of DPF system, however, when an exhaust gas temperature is low, a temperature of the DOC decreases such that the DOC becomes inactive, and therefore an oxidation reaction is not promoted. Hence, DPF regeneration through PM oxidation cannot be performed, and as a result, PM continues to accumulate in the DPF, leading to increasing filter blockage.
In response to the filter blockage, when a PM accumulation amount exceeds a predetermined accumulation amount, the PM collected in the DPF is forcibly burned and removed by forcibly raising the exhaust gas temperature to a target temperature (between approximately 500° C. and 600° C., for example).
The PM accumulation amount is commensurate with an output value of a differential pressure sensor that measures a differential pressure of the exhaust gas between a front side and a rear side of the DPF. Therefore, when the output value of the differential pressure sensor exceeds a predetermined differential pressure, an ECU (Engine Control Unit) detects a filter blockage, whereupon the ECU either starts DPF regeneration automatically (automatic regeneration) or, in a case where DPF regeneration is to be performed manually (manual regeneration), illuminates a DPF warning lamp (a DPF lamp) provided in a cabin to prompt a driver to start DPF regeneration by pressing a regeneration execution switch (a manual regeneration execution switch) after stopping a vehicle.
Note that filter blockage detection may be performed on the basis of a traveled distance rather than the differential pressure of the exhaust gas between the front and rear sides of the DPF. In this case, DPF regeneration is started automatically or manually, as described above, when the traveled distance exceeds a predetermined distance.